Three-dimensional integral display using plastic opticalfibers

Youngmin Kim, Heejin Choi, Seong-Woo Cho, Yunhee Kim, Joohwan Kim, Gilbae Park,and Byoungho Lee*

School of Electrical Engineering, Seoul National University, Kwanak-Gu Shinlim-Dong, Seoul 151-744, Korea

*Corresponding author: byoungho@snu.ac.kr

Received 9 April 2007; revised 23 July 2007; accepted 9 August 2007;posted 21 August 2007 (Doc. ID 81916); published 4 October 2007

A novel approach to an integral imaging system using a pliable plastic optical fiber array is proposed. Theproposed system has the advantage that it can utilize a light source for three-dimensional (3D) imagesat an arbitrary location because the point light sources are formed by the plastic fiber array with flexibleoptical paths. Two-dimensional images can also be expressed in the proposed system. The light efficiencyof this system is high compared with previous point light source array integral imaging systems. Thefeasibility of the proposed method is explained and demonstrated with experiments. © 2007 OpticalSociety of America

OCIS codes: 110.2990, 100.6890.

1. Introduction

It is well-known that optical fibers are used in fiber-optic communication, which permits digital datatransmission over long distances at higher datarates than other forms of wired and wireless com-munications. Optical fibers are also widley appliedto sensor technologies, and in some applicationsthey are used for lighting and display. With therapid advances in electronic devices and modules,the display technique has also progressed impres-sively. Recently the next generation display tech-nologies, such as organic light emitting diodes,electronic papers, and three-dimensional (3D) dis-play have been researched by many enthusiasticinvestigators. In spite of the endeavors of the de-voted researchers, the techniques in these fieldsstill need further investigation. Among them, thereason 3D display attracts special attention is thatthis is the only technique that can realize a naturalviewing perspective. It has attracted much atten-tion for use in advertisements, education, virtualreality, entertainment, medical systems, militaryapplications, aerospace applications and automo-

tive applications. There are many categories in 3Ddisplay, among which integral imaging, which isalso called integral photography, shows a promisingway to produce a practical 3D display [1,2]. It hasthe advantages of full parallax, color images, andquasi-continuous viewpoints within a viewing an-gle, and does not require the use of any specialglasses [3–6]. On the other side, it has limited view-ing angle, depth, and image resolution. To alleviatethem, some novel methods based on integral imag-ing with ameliorated depth and viewing angle werereported recently [7,8]. The 3D display using a pointlight source array [9–13] is an important conceptbecause its resolution can be independent of theimage depth and is mainly determined by the num-ber of point light sources.

With these approaches, the 3D to two-dimensional(2D) convertible display systems have attracted greatattention. Because consumers are so accustomed tothe mature 2D display technology, many people de-sire 3D display to have quality as good as 2D displays.However, in most technologies, if we hope to display3D images, we cannot help sacrificing some quality of2D images. Therefore, in general, the 3D image res-olution is inferior to the 2D image resolution. And 3Ddisplay contents have not been much developed yetcompared with 2D contents. Consequently, we need

0003-6935/07/297149-06$15.00/0© 2007 Optical Society of America

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3D-2D convertible display systems for these reasons.The 3D-2D convertible display system will also be ofgreat help in establishing the 3D display market.

In the original work of the point light source inte-gral imaging [11] collimated light through a lens isused for producing a point light source array. How-ever, in this case, we should adjust the gap betweenlens array and display panel elaborately and the totalsetup has a bulky structure, while it has sublimeoptical efficiency. Even though direct generation of apoint light source array, such as a pinhole array, wasproposed, the structure leads to serious optical effi-ciency deterioration [12].

In this paper, to improve light efficiency, we pro-pose the concept of the integral imaging system usinga pliable plastic optical fiber array as a 3D lightsource. To the best of our knowledge, it is the firstreport where optical fiber is used for 3D integral im-aging. In the proposed system, the light source forexpressing the 3D image can be placed at an arbi-trary position since the plastic optical fibers are pli-able. The coupling of light to a fiber array can bedone remotely, separated from the display panel, andhence it is easy to control the coupling efficiency.

2. Principle of the Proposed Method

The basic principle of the proposed system is shownin Fig. 1. The proposed system consists of a plasticoptical fiber array, an acryl plate with holes to fix thefiber array, a backlight source, and a spatial lightmodulator (SLM). The plastic optical fiber array isbendable so that the light source for 3D display can belocated at a separate location, and the end of it is fixedwith uniform spacing. Consequently the plastic opti-cal fibers play the role of waveguides. The large ap-erture of the plastic multimode optical fiber makesthe light coupling easy and light efficiency high.Moreover the cross sections of the fibers are small,which is tantamount to a dense arrangement so thatthe fiber bundle can be coupled to a light source to-gether. Hence the light emitting area of the sourcecan be smaller than the display panel size showing3D images. Figure 2 shows in more detail the struc-ture of the plastic optical fiber output plane (which isused as the point light sources) and the acryl platewith holes to hold a plastic fiber array.

In our system, the expression of 2D images is alsopossible. A flat backlight unit (BLU) can be used as inFig. 1(b) for 2D display. In this case, the plastic op-tical fiber array and acryl plate can be regarded asunnecessary elements. Optical efficiency can be dete-riorated by acryl plate when the light is transmittedthrough it, and the light can be occluded by plasticoptical fibers fixed on the acryl plate. However anacryl plate is a transparent material so as to transmitthe parallel rays and it serves just like a weak dif-fuser. Due to this diffusing property, the effect of theocclusion of the light by fibers can be mitigated. Alsothe arrangement of the plastic optical fiber array issparse. Therefore we can acquire vivid 2D images.

In integral imaging, a 3D image is integrated by 2Delemental images. Therefore the 3D image quality is

limited by 2D elemental images. The figure of meritfor the 3D image is expressed as a function of thenumber of point light sources and the pixel pitch of anSLM:

� � NPLS � SEI��SLM, (1)

where � is the figure of merit for 3D image, NPLS isthe number of point light sources, SEI is the size ofeach elemental image region, and �SLM is the pixelpitch of an SLM. The resolution of the 3D image isdetermined by the number of point light sources. Thenumber of views by point light sources is determinedby the size of elemental image divided by the pixelpitch of an SLM [14]. A detailed principle of recon-structing 3D images with the point light source arrayis shown in Fig. 3. As shown in Fig. 3(a), a unitconsisted of a point light source and the correspond-ing region on the SLM is one pixel that emits andmodulates light rays with different intensities ac-cording to the observation directions. Because thefunction of the SLM is only the formation and mod-ulation of the point light sources, the key for deter-mining the resolution of the generated 3D images isthe number of point light sources. The real lines in

Fig. 1. (Color online) Schematics of the proposed method consist-ing of an SLM, an acryl plate, a plastic optical fiber array, and lightsources: (a) the case of generating point light source array (3Dmode), (b) 2D mode.

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Fig. 3(a) connect point light sources and the eye ob-serving an integrated image. The pixels of the SLMon those lines are modulated to represent a 3D imageas shown in the figure. Another important factor inthe principle of integral imaging is binocular dispar-ity, which will be explained later in this paper. There-fore, as can be figured out from Fig. 3(a), the imageresolution is determined by the number of point lightsources [14]. The dotted lines in Fig. 3(a) are exam-ples of the regions unrecognized by the eye becausethere are no rays corresponding to the lines frompoint light sources. The SLM pixels assigned to anelemental image area for a single point light sourceare used to provide different viewing directions withappropriate modulation of the light. Hence, when thesize of the elemental image for each point light sourceis fixed, the number of views can be increased by use

of an SLM having a higher resolution. Each pixel ofan SLM expresses the information to represent thereconstructed 3D images. Figure 3(b) depicts the

Fig. 2. (Color online) Structure of (a) fiber output plane that isused as the point light source, (b) the schematic of an acryl plate(0.27 mm hole size and 1 mm spacing).

Fig. 4. (Color online) Difference between the focused light sourceand extended light source.

Fig. 3. (Color online) Forming a 3D image with a point lightsource array: (a) resolution limitation, (b) principle.

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principle of the reconstructed 3D image that is inte-grated from several elemental images on the displaypanel.

Unlike the focused light sources [11,13], ourmethod does not provide an ideal point light sources.It provides an extended light source array becausethe core diameter is �0.27 mm. The light coming outof each multimode optical fiber can be modeled as asuperposition of point light sources over the fiber corearea. Each ray diverges in an arbitrary directionwithin acceptance angle of the fiber as shown in Fig.4. As we can see in Fig. 5, adjacent rays in the opticalfiber core can affect the integration of 3D image. Be-cause the extended light source has a diameter largerthan the pixel pitch �0.036 mm� of an SLM, theblurred 3D image can be generated by many adjacentrays in the optical fiber core. Hence for consideringthe blurring effect by adjacent rays, the simulation isexecuted with ray optic analysis. The extended lightsources are located at 3 mm behind of SLM and theletter “P” is formed at 20 mm in front of the fiber

facet. The observer is located at 400 mm in front ofelemental image (SLM) and the number of elementalimages is 40 � 40. Figure 6 shows the simulationresults. Unlike the focused light source, we observethe results of the blurring effect.

3. Experimental Results and Discussions

We demonstrated our proposed system experimen-tally with the setup of Fig. 7. We used an SLM as atransmission-type display panel that had a 0.036 mmpixel pitch and a 37 mm � 28 mm active area. Theacryl plate has 40 � 40 apertures, fabricated by adrilling machine, each of which has a diameter of0.2788 mm. The space between apertures is set to be1 mm and the thickness of the acryl plate is 3 mm.The whole acryl plate size is 9 cm � 9 cm, and it istransparent so as to transmit surface light from BLU,while it plays the role of a frame for plastic opticalfibers. The plastic fiber chosen in our system wasfabricated by Nuvitech Corporation; it is step indexplastic optical fiber with a polymethyl methacrylate(PMMA) core and fluoric high-material cladding. Itsdiameter is larger than that of other optical fibers andis highly flexible and resistant to vibration or bend-ing. Also it is much cheaper than the glass opticalfiber. The length of each plastic optical fiber is�15 cm. With this acryl plate and plastic optical fi-ber, we manually inserted fibers into the holes in theacryl plate. The measured diameter of plastic opticalfibers is in the range of 0.238–0.270 mm. The corecross section is 98% of the overall cross sectional areaof the plastic optical fiber. Hence the core diameter is0.2330–0.2646 mm and the numerical aperture is0.38; the fibers are fixed side by side on the holes ofthe acryl plate with instant adhesive. For regularspacing of the point light source array, the ends offiber array should be well-arranged without deterio-ration. The resolution of the 3D image is 40 � 40. Thelight source that is used for the plastic optical fiberarray is a high-intensity illuminator by Fiber-LiteCorporation (MI-150). The BLU used for 2D display isa commercial incoherent flat white light source thathas a luminance of 4000 cd�m2 and is mostly used for

Fig. 6. (Color online) Simulation results of the focused lightsource and extended light source �dels denotes the diameter of theextended light source). Fig. 7. (Color online) Experimental setup.

Fig. 5. (Color online) Blurring effect by adjacent rays in the op-tical fiber.

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mobile LCDs. No other special technique is used forthe BLU in the experiments to improve optical effi-ciency of BLU in itself. The SLM is located in front ofthe BLU and the acryl plate with plastic fiber array islocated between the SLM and the BLU as in Fig. 1.The experimental results to verify the functions oflight sources of the proposed system are shown in Fig.8. Figure 8(a) shows generation of the point lightsource in 3D mode. By controlling the intensity of theilluminator, the optical efficiency of the 3D image canbe regulated. Figure 8(b) shows the surface lightsource in 2D mode. In our proposed method, the fullwhite luminance with the SLM can be controllablein the 3D mode and it is 3600 cd�m2 in the 2D mode.These results show superior efficiency compared withthe previous case in which 25% of total light wastransmitted [13]. The efficiency of the 3D mode canalso be augmented by increasing the number of pointlight sources on the acryl plate or by compact spacingof the plastic optical fibers in the array.

For 3D display, two letters are integrated: “3” and“D” are formed, respectively, at 20 mm in front of andbehind the point light source array that was imple-mented by the pliable plastic optical fiber array. Theexperimental schematic is shown in Fig. 9. Figure9(a) shows how to form elemental images for real andvirtual images. For a real image 3, at the cross pointsof the real lines (that connect point light sources andthe integrated image) and the SLM plane, elementalimages for the image 3 are provided. For a virtualimage D, at the cross points of the dotted lines (whichare extensions of the lines that connect the point lightsources and the virtual image D) and the SLM plane,elemental images for the image D are provided. Whenthe right and left eyes observe the displayed images,the images 3 and D appear in front of and behind thedisplay panel due to the binocular disparity, as canbe seen in Fig. 9. The experimental results with theproposed system are shown in Fig. 10. We can see fivedifferent perspectives according to viewing directionsin Fig. 10(a). Figure 10(b) shows 2D images that aredisplayed on the SLM and captured by the CCD cam-era. The 2D images are displayed with full resolution�1024 � 768�. Although the acryl plate is transpar-ent, there exist some defects in the 2D images due toocclusion of the BLU by the fiber cores. However, the

fibers are sparse enough to transmit the backlightsource through the gaps between adjacent fibers andthe acryl plate has a diffusing property. Hence we canobtain good quality 2D images as in Fig. 10(b). In

Fig. 8. (Color online) Pictures of the acryl plate when the lightsfor the (a) 3D mode and (b) 2D mode are on.

Fig. 9. (Color online) (a) Formation of elemental images for thereal and the virtual images, (b) the binocular disparity.

Fig. 10. (Color online) Experimental results: (a) 3D images ob-served from different viewing directions, (b) 2D images.

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addition, the instant adhesive material is stuck at theends of the plastic optical fiber array because of themanual process, which induces blurring of the im-ages. This can be mollified by a fine process of usingadhesive and adopting plastic optical fibers having auniform size.

4. Conclusion

We proposed a novel optical-path-flexible integral im-aging system by using a pliable plastic optical fiberarray. The proposed system uses a light source for 3Dimages from an arbitrary location because the pointlight sources are formed freely by the plastic fiberarray regardless of optical paths. Our display systemcan display both 2D and 3D images using differentlight sources and the optical efficiency of 2D images isameliorated. We expect that the optical fibers canalso be used with flexible devices, i.e., they may pro-vide a variety of flexible configurations in integralimaging if the flexible display panel itself is devel-oped.

This research was supported by a grant (F0004190-2007-23) from the Information Display R&D Center,one of the 21st Century Frontier R&D Programsfunded by the Ministry of Commerce, Industry andEnergy of the Korean Government.

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